While rigid clear plastic canisters have existed in the art of materials packaging for some time, several difficulties with their production and use still exist.
A first problem lies in preserving the clarity of the canister. While clear plastic canisters are easily and rapidly produced by commonly known blow molding and extrusion techniques, the sidewalls tend to have flow marks and irregularities in thickness which distort or obscure the view of the materials inside. Such distortion is undesirable because the packaging is often intended to enhance the appearance of the product inside, and hence increase its desirability to consumers.
A second problem lies in labeling the canister. It is frequently desirable that the canister be labeled before it is filled with product so that the processing of the packaged product is fully completed once the canister is filled. Several advantages are realized by filling a pre-labeled canister: no additional steps in packaging are needed once the canister is filled, and the packaged product can go directly into the market place; the labeled canisters used for packaging can be pre-prepared at different locations and shipped with lower weight, and thus lower shipping costs; and mislabeled or defective canisters can be detected at a stage of processing where the costs of correction are lower. Unfortunately, there is no effective way to pre-print the component stock for blow molded or extruded canisters so that the canister emerges from the blow molding or extrusion process with a label on it. These methods mechanically deform the plastic component stock to form the canister. Canisters produced by methods such as heat-shrinking the thermoplastic component stock to form a canister (e.g., as shown in U.S. Pat. Nos. 4,053,346, 4,072,549, and 4,559,765), cannot be pre-labeled either, due to problems with distortion of the stock. However, there can be difficulties with the application of a label even after the canister is formed.
If the labeling is to be added by embossing, impressing, printing, or painting the sidewalls of the finished canister rather than using a sticker, the process becomes difficult and expensive due to the curved (and possibly nonrigid) work surface that the canister sidewalls provide. A rounded, flexible work surface is difficult to support during the labeling process without the use of complex machinery. It is also difficult to devise a process which will apply the label at the same position on the sidewalls or the canister bases of all the canisters. This is a particularly important consideration where a multicolored label is to be printed on the canisters; in such a case, when the colors are printed on the canister one by one, each color pattern must be precisely aligned with the color pattern applied by the previous printing step or a distorted label will result. These same considerations apply to canisters which are produced by methods other than blow-molding and extruding.
Another problem lies in devising a manufacturing process which can be quickly and easily adapted to produce canisters of different height and diameter as the need requires. Blow molded and extruded canisters need different molds and dies if the canister size is to be changed, and changing these parts may require an extensive shutdown of the manufacturing process while the new parts are installed. Other manufacturing methods generally involve wrapping a sidewall blank about a mandrel and a canister base and welding the seams (e.g., as shown in U.S. Pat. Nos. 4,041,848, 4,360,838, 4,334,875, 4,349,575, 4,745,535, and 5,135,462). These methods are often unsatisfactory; besides being generally directed towards thermoplastic-coated cardboard materials, they require a change in numerous parts in the manufacturing apparatus if the canister size is to be changed, and therefore they too require substantial time to change over.